In a thermal printer such as described in U.S. Pat. application Ser. No. 457,593, filed Dec. 27, 1990 in the names of S. Sarraf, et. al., and assigned to the same assignee of the present patent application, there is described a system which uses the heat energy of a laser beam to print a finely detailed image onto a receiver element, which for example is a slide transparency. The printed image, which should be as faithful as possible to an original or master image, is printed as a series of dots or pixels by scanning the laser beam a line at a time across a dye donor element held by vacuum against a blank slide transparency or receiver element. The laser beam is rapidly modulated in intensity by input data corresponding to the master image and "spots" or pixels of dye, corresponding to the pixels of the master image, are transferred by thermal energy from the dye donor element onto the slide transparency. To obtain sufficiently high resolution, such as is needed in photographic slide transparencies to give a desired degree of sharpness in the projected image (e.g., at 100 power magnification), the dye pixels are very small (e.g., only about 7 microns) and are written at a very close pitch, for example, about 4000 dots per inch. They are written at a high speed (e.g., 140 KHZ) since otherwise the printing of the entire image takes an inordinately long time.
As explained above, for a thermally printed reproduction of a master image to be suitable for viewing at projection magnification (e.g., 100 power), it is necessary that the individual pixels of the printed image be very small and very accurately positioned. And it is also highly desirable that the individual pixels each have a density or tone value corresponding as closely as possible to the respective densities of the pixels of the master image. If otherwise, the tones and contrast of the printed image are distorted to a greater or lesser degree and visual quality of the image is degraded.
In the kind of thermal dye-transfer imaging described above, there is employed a dye donor element in the form of a thin sheet of material having a thermally reactive dye on one surface. Such a donor element is disclosed in U.S. Pat. No. 4,973,572 and assigned to the same assignee of the present patent application. The donor element is placed with its dye coated surface closely adjacent (e.g., about 8 microns distant) to a receiver element, such as a blank slide transparency. Then the donor element is "scanned" by a laser beam focused on the back of the element to a very small spot of light (e.g., about 6 microns). As the laser spot is linearly scanned across the donor element, the laser is electronically modulated in intensity at very high frequency to provide greater or lesser heat energy in the focused light spot. The thermal energy in a light spot passing through the donor element causes the dye, over the area of the spot, to ablate or to sublime to a greater or lesser degree depending on the heat energy content of the light. The dye thus removed in the area of the light spot transfers as a dot or pixel of dye printed onto the receiver element (slide transparency). The density of such a transferred dot of dye is a function of the thermal energy absorbed through the donor element into the dye from the light spot.
It has been found that the density of a pixel of dye in a thermal printer of this kind after being printed on the receiver element is not linearly related to the intensity of the laser beam spot. As a result, the tone scale of the printed image is not a linear reproduction of the tone scale of the master image. This results in a degradation of the visual quality of the printed image, especially when viewed at projection magnification. Moreover, attempts to improve the linearity of tone scale have been frustrated by minor variation in the thermal reactivity of the dye over the surface of a donor element or variations from one dye donor element to another. Thus steps taken to improve the linearity of tone scale as printed by an intensity modulated laser have not been fully successful. Furthermore, there are certain inherent non-linearities in the operation of a laser which make doubly difficult the control of its intensity and focusing of its light spot as used in a thermal printer such as described above
It is desirable to overcome the above described difficulties and limitations and make possible the printing of thermal dye images with substantially improved linearity in tone scale and in full color. Slide transparencies so produced would closely approach or equal in visual quality a photographic color image even when viewed at projection magnification.